Experiments are in progress to characterize distinct rabbit lymphoid cell populations, to identify cell surface markers and patterns of gene expression for them and to relate these populations to cell function. Attempts to characterize a CD4 homolog to serve as a marker for the T helper subset of rabbit lymphocytes have progressed in this period. A gene fragment was amplified from rabbit thymus RNA using primers based on sequences of primate and rodent CD4 molecules. This fragment was used to screen a cDNA library and a clone with appropriate characteristics was found. The data available for codons 65 to the 3' untranslated region reveal over 75% sequence identity to human CD4 and about 68% identity to rodent L3T4 molecules. A search is underway for a full length clone; when this is available it will be expressed in mouse cells and monoclonal Ab against it prepared. In addition, genes encoding other molecules of interest to these studies, such as IL-2 and IL-2r are being cloned from cDNA libraries. Studies of lymphoid cell characterization have concentrated on gut associated lymphoid tissue. These studies are given impetus by the fact that HTLV-l infection is readily passed by milk from infected mothers. A number of antibodies are being tested for reactivity with GALT, thymus, fetal liver and bone marrow in young animals to trace developmental pathways and identify T and B cell percursors in the rabbit lymphoid system.